New Accounting Standards (Policies)	6 Months Ended
New Accounting Standards (Policies)	Mar. 31, 2021
Accounting Policies [Abstract]
New Accounting Standards	New Accounting Standards Accounting standards implemented during fiscal year 2021 On October 1, 2020, the company implemented Accounting Standards Update ("ASU") 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. The ASU introduces an approach based on expected losses to estimate credit losses on certain types of financial instruments, including accounts receivable. The ASU also modifies the impairment model for available-for-sale debt securities and provides for a simplified accounting model for purchased financial assets with credit deterioration since their origination. The guidance had an impact on the company's accounting policies and procedures related to calculation of allowance for doubtful accounts receivable but did not have a material impact on its Condensed Consolidated Financial Statements. On October 1, 2020, the company implemented ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement. The amendments in this ASU add, modify, and eliminate certain disclosure requirements on fair value measurements in Topic 820. The guidance did not have a material impact on the company's Condensed Consolidated Financial Statements Accounting standards to be implemented Implementation of the following standards may result in a significant change in practice and/or have a significant financial impact on the company. In August 2020, the FASB issued ASU 2020-06, Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging—Contracts in Entity’s Own Equity (Subtopic 815-40): Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity (Subtopic 815-40). The ASU simplifies the accounting for certain financial instruments with characteristics of debt and equity, including convertible instruments and contracts on an entity’s own equity. ASC 470-20 outlines five models to allocate the proceeds attributable to the issuance of a convertible debt instrument. This ASU removes from U.S. GAAP the separation models for convertible debt with a cash conversion feature (CCF) and convertible debt with a beneficial conversion feature (BCF). As a result of adopting this ASU, entities are not required to separately present in equity an embedded conversion feature in such debt. Instead, they should account for a convertible debt instrument wholly as debt. Applying the separation models in ASC 470-20 to convertible instruments with a CCF or BCF involves the recognition of a debt discount, which is amortized to interest expense. The elimination of CCF and BCF models will reduce reported interest expense and increase reported net income for convertible instruments issued within the scope of those models before the adoption of ASU 2020-06. The amendments in this ASU are required to be adopted by public business entities in fiscal years beginning after December 15, 2021, including interim periods within those fiscal years. Early adoption is permitted, but not earlier than for a fiscal year beginning after December 15, 2020, including interim periods within those fiscal years. Entities are permitted to adopt the guidance through either a modified retrospective method of transition or a fully retrospective method of transition. The company is currently evaluating the potential impact of this guidance on its accounting policies and its Condensed Consolidated Financial Statements.
Inventories	Inventories Inventories are stated at the lower of cost (using FIFO or average methods) or market (determined on the basis of estimated realizable values)
Fair Value	Fair Value FASB guidance provides a fair value hierarchy that prioritizes the inputs to valuation techniques used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical instruments (Level 1) and the lowest priority to unobservable inputs (Level 3). The three levels of the fair value hierarchy are described below: •Level 1 inputs use quoted prices in active markets for identical instruments. •Level 2 inputs use other inputs that are observable, either directly or indirectly. These Level 2 inputs include quoted prices for similar instruments in active markets and other inputs such as interest rates and yield curves that are observable at commonly quoted intervals. •Level 3 inputs are unobservable inputs, including inputs that are available in situations where there is little, if any, market activity for the related instrument. In instances where inputs used to measure fair value fall into different levels in the above fair value hierarchy, fair value measurements in their entirety are categorized based on the lowest priority level input that is significant to the valuation. The company's assessment of the significance of particular inputs to these fair value measurements requires judgment and considers factors specific to each asset or liability.
Environmental	Environmental Federal, state and local requirements relating to the discharge of substances into the environment, the disposal of hazardous wastes and other activities affecting the environment have, and will continue to have, an impact on the operations of the company. The process of estimating environmental liabilities is complex and dependent upon evolving physical and scientific data at the sites, uncertainties as to remedies and technologies to be used and the outcome of discussions with regulatory agencies. The company records liabilities for environmental issues in the accounting period in which they are considered to be probable and the cost can be reasonably estimated. At environmental sites in which more than one potentially responsible party has been identified, the company records a liability for its allocable share of costs related to its involvement with the site, as well as an allocable share of costs related to insolvent parties or unidentified shares. At environmental sites in which Meritor is the only potentially responsible party, the company records a liability for the total probable and estimable costs of remediation before consideration of recovery from insurers or other third parties.

Disclosure of inventory accounting policy for inventory classes, including, but not limited to, basis for determining inventory amounts, methods by which amounts are added and removed from inventory classes, loss recognition on impairment of inventories, and situations in which inventories are stated above cost.